The present invention relates to the application of the photorefractive effect to the fabrication of optical devices based on oxynitride glass, and more particularly, to photoinduced Bragg gratings in oxynitride optical fibers.
Reflection gratings are often implemented as waveguides which have a path region having a modulated refractive index. The waveguiding structure is often in the form of a fiber. The modulation preferably takes the form of alternate regions of higher and lower refractive index. These periodic variations in refractive index act as a Bragg grating, and they selectively reflect light having a wavelength of twice the spacing. Such gratings can be used to filter, to define laser cavities and as components in multiplexers and demultiplexers.
Photoinduced Bragg gratings have been made in a variety of ways. One approach, which is disclosed in U.S. Pat. No. 4,725,110, is to direct two interfering beams of ultraviolet radiation through the cladding of an optical fiber to form an interference pattern along the germania-doped glass core. Other techniques involve subjecting regions of a fiber core to ultraviolet radiation through an amplitude mask or a phase mask. U.S. Pat. No. 5,287,427 teaches that the refractive index effect is enhanced by exposing that part of the glass that is to be subjected to actinic radiation to hydrogen or deuterium.
Germania-doped silica has shown the greatest refractive index change (.DELTA.n) after being subjected to actinic radiation. For various reasons attempts have been made to make gratings from photosensitive materials other than germania, a relatively scarce, expensive constituent. An object of the invention is to provide reflective gratings that are formed from commonly occurring, inexpensive materials. Another object of the invention is to provide a germania-free glass from which reflection gratings can be made.
Reflective gratings have been made from other UV sensitive oxides that are less effective than germania. It is disclosed in WO 94/00784 that photoinduced ratings can be made from B.sub.2 O.sub.3 in combination with SiO.sub.2 or GeO.sub.2. The publication, Kitagawa et al., OFC Vol.4 of 1994 OSA Technical Digest Series, paper PD-17 teaches that gratings can be made by pulsing optical fibers having P.sub.2 O.sub.5 --SiO.sub.2 cores with 193 nm radiation. U.S. Pat. No. 5,478,371 teaches a technique for forming gratings in P.sub.2 O.sub.5 doped optical fiber with 248 nm radiation. Such photosensitive oxides can be used alone or in combination with other photosensitive oxides such as germania. A further object of the invention is to provide a photosensitive material that can be used in combination with other photosensitive materials to form reflection gratings.